It seems to be soap and/or dirt (or perhaps hard water from the pre-rinse) that comes out of crevices when you rinse. If you can deal with the hassle/expense of changing out the CR filters often, you can try an extended rinse (or pressure washer, which I haven't done). My technique was to rinse with the CR, then blow out the crevices (since you have to do this anyway) and then while I was at it, blow off the rest of the car, which leaves a few drips to clean up with a towel.
This worked pretty well, but after a while it seemed like it made more sense to just do my whole wash with soft water from in the house, since I was blowing the rinse water, anyway. I'm sorry I seem to be rambling through this...but the CR, although very helpful, never got me to a "rinse and walk away" process because of the water in the cracks and crevices.
